Answer
The mirror helps the customer to see behind herself.
Work Step by Step
The convex mirror allows customers to see a wide-angle view behind themselves to see if someone is sneaking up.
The radius of curvature is twice the focal length, and considerations that affect f include how wide of a view is desired. For example, if the ATM is at the end of a long hallway, the rear view doesn’t need to be all that wide.
